1. Field of the Invention
This invention relates to pushing and pulling implements of the fluid pressure type, such as hydraulic jacks. More specifically, the invention concerns a mechanical, self-locking mechanism for holding loads after the hydraulic jack has been placed in a load supporting position.
2. Description of the Prior Art
Hydraulic jacks have been used as outrigger supports for mobile cranes to provide stabilizing support to permit lifting of heavy loads by the crane at the job site. Various means have been used to lock the jacks in an extended position supporting a load. Such means include both hydraulic and mechanical devices.
One type of hydraulic lock consists of a pilot operated check valve that is located at the pressure port of the jack cylinder for locking the hydraulic fluid within the cylinder. When pressure is applied to the cylinder in a jack raising direction, the check valve will unseat and admit more fluid to the cylinder. If pressure is applied to lower the jack, the pressure acting upon a pilot spool will unseat the check valve, to allow the discharge of fluid from the cylinder and the jack to retract. Another type of hydraulic lock is a shut-off valve that is located at the pressure port of the jack cylinder, so that closure of the shut-off valve, either manually, hydraulically or electrically, will block the hydraulic fluid in the cylinder. While such hydraulic locks provide a degree of security against inadvertent lowering of the jack, they do not insure against lowering of the jack due to seepage or leakage of hydraulic fluid, as might occur past the locking valve or from the jack cylinder.
One type of mechanical lock, to prevent the inadvertent retraction of outrigger jacks, consists of locking pins. These pins are inserted into a series of holes, that are located in the jack housings and in the members that are connected to jack rams, so that locking positions can be obtained at discrete intervals through the extension travel range of the jack. This system provides secure locking independently of the hydraulic system. Such locking pins have the disadvantage of requiring manual placement and removal at each jack location. These pins are subject to being lost. The hole spacing for the pins is discrete and requires an "inching" adjustment that is not compatible with random jack extensions. Furthermore, the application of this mechanical lock is left to the option of the crew operating the crane.
Another type of mechanical lock for maintaining a hydraulic outrigger jack in an extended position is a screw lock, as shown in U.S. Pat. No. 3,702,181. A shaft is journalled at the top of a hydraulic cylinder and extends coaxially therein, where it is keyed to a threaded locking member. The shaft is rotated by a handle, located above the cylinder, and upon rotation of the shaft, the locking member moves to a locking position that is located between the piston and one end of the cylinder. The locking member can either be threadedly mounted within the piston and adapted to engage one end of the cylinder or it can be threadedly mounted within one end of the cylinder and adapted to engage the piston. This lock requires a manual setting at each jack location and its application is left to the option of the crew operating the crane.
Another type of screw lock for a hydraulic cylinder is shown in U.S. Pat. No. 2,875,980. A screw, coaxially located within a hydraulic cylinder, threadedly fits through a piston and into a hollow piston rod to lock the piston in a desired position within the cylinder. The screw is connected to a vertical shaft that enters the cylinder through a packing gland. A bevel gear is fixed to the shaft outside of the cylinder and a thrust bearing is fixed to the shaft within the cylinder to hold the shaft in a fixed axial position relative to the cylinder. The bevel gear and shaft are turned by another bevel gear that is mounted upon a shaft. This shaft is turned either manually, by a crank, or automatically, through a suitable drive with an electric motor that is energized when the hydraulic cylinder is actuated. The packing glands provide an additional location for possible leakage of hydraulic fluid from the cylinder.
Another type of screw lock is shown in U.S. Pat. No. 2,284,958. A hydraulic cylinder has a piston therein with an externally threaded piston rod that extends upwardly through the top of the cylinder to a load supporting pad at the upper end of the rod. A nut, that is located on the piston rod portion outside of the cylinder, can be manually adjusted, after the piston rod is in a desired load supporting position, to bear against the top of the cylinder and thereby lock the piston rod in that position. This type of lock requires manual setting and manual releasing at each jack cylinder location and its application is left to the option of the person operating the jack.
The use of a driven worm for turning a worm wheel to raise or lower a jackscrew is shown in U.S. Pat. Nos. 2,234,220; 3,236,489; 3,790,133; and 3,888,464.